Conventionally, apparatuses which move or rotate an object and, when the object passes a predetermined position or angle, output a coincidence signal are known. These apparatuses detect a displacement or an angle by an electrostatic capacitance sensor or an encoder.
An electrostatic capacitance sensor outputs direct displacement or angle information continuously. Hence, the information can be converted into, e.g., a voltage value to directly compare the current position or angle of the object with the predetermined position or angle.
When an encoder is used, the original period or zerocrossing of the encoder is counted, and accordingly, the current position or angle of an object can be directly compared with a predetermined position or angle.
For an accurate application purpose, the original period is interpolated to increase the resolution.
Conventionally, an analog interpolation scheme has been used for these application purposes.
In the analog interpolation scheme, a periodical signal is output continuously. Hence, when the periodical signal is counted, the current position or angle of an object can be compared with a predetermined position or angle.
In the analog interpolation scheme, the periodical signal of an encoder is converted into a period shorter than the original period.
FIG. 6 is a view for explaining the prior art by exemplifying an encoder.
A 2-phase output 2 is output from an encoder 1 as an object moves or rotates. The 2-phase output 2 contains sinusoidal-wave-shaped signals 90° out of phase. The 2-phase output 2 is electrically processed by an analog interpolator 30 to generate some periodical signals 31 in one period. In this example, one period is interpolated into four parts.
The analog interpolator continuously outputs a signal in accordance with the movement. This signal is counted by a counter 6 to obtain a signal 32. When a target 9 of coincidence detection is at the Nth pulse, a coincidence detection signal is output at a timing 11.
As described above, when the output from the analog interpolator is counted, coincidence detection can be done at the resolution of the analog interpolator.
However, for more accurate coincidence detection, e.g., to increase the number of interpolation parts to 1,000 or more, the analog interpolation scheme requires a larger circuit scale that is difficult to implement because of cost and labor for signal adjustment.
In addition, in the analog interpolation scheme, the final resolution is counted. For this reason, if the number of interpolation parts is increased, the number of digits of the counter increases. Furthermore, even when the speed is unchanged, the frequency increases in proportion to the resolution. Accordingly, the counter itself must increase the speed.
As described above, the analog interpolation scheme has two problems.
To solve these problems, a digital interpolator (scheme) which interpolates a period by digital signal processing has been proposed.
A digital interpolator is constituted by an analog/digital converter and a digital signal processing section. Hence, even when the number of interpolation parts is increased, the circuit scale does not increase.
When the digital interpolator is combined with a control unit, and the performance in the stationary state is required by a positioning unit, the digital interpolator can be used like an analog interpolator.
However, the digital interpolator obtains displacement or angle information for every sampling and therefore cannot obtain information between sampling.
This is inconvenient for accurate coincidence detection.